In recent years, the prevalence of acquired immunodeficiency syndrome (AIDS) has been rapidly increased, and there is worldwide need for establishing methods for its prophylaxis and treatment. AIDS is caused by human immunodeficiency virus (HIV), a Lentivirus in the family Retroviridae. For HIV, at present, there are known two types of viruses, HIV-1 and HIV-2. HIV mainly infects CD4(+) T-lymphocytes in humans, anthropoid apes (chimpanzees), and others, and destroys them and causes AIDS.
As agents against AIDS are currently used inhibitors of reverse transcriptase characteristic of HIV [for example, zidovudine (AZT, ZDV), didanosine (ddI), zalcitabine (ddC)] and of viral proteolytic enzymes (for example, saquinavir) Actually, two types of agents having different mechanisms of action are combined. Although these are intended to kill HIV-1, they display side effects and may lead to the generation of viruses that are resistant to them by taking them over a long period of time, for example, and thus it cannot always be said that these agents are satisfactory ones.
The genome of a retrovirus has a major characteristic in that the genome is integrated into a chromosome of an infected cell through the phenomenon of reverse transcription. Accordingly, it is impossible for agents and others to eradicate the virus throughout the body, and thus infected individuals necessitate co-existence with the virus in their life.
Another characteristic of retroviruses is that they possess genes encoding for a group of proteins referred to as accessory proteins (for example, Nef, Vif, Vpr, Tat, Rev), in addition to genes for proteins constituting a virus (for example, Gag, Pol, Env). Among them, Vpr is a small protein consisting of 96 amino acids and has a variety of functions of, for example, transporting to the nucleus a viral genome-protein complex, designated as pre-integration complex (immature virus); arresting the cell cycle of infected cells at the G2/M period, thereby enhancing the efficiency of transcription of the virus genome within nondividing cells such as macrophages; destroying surrounding cells (bystander cells) in lymph nodes via cell-membrane impairing activities. In fact, it is known that HIV-1 genome is actively replicated when an infected cell is at the G2/M period, and that an HIV-1 mutant deficient in Vpr results in extremely reduced production of mature viruses in macrophages.
It is also reported that Vpr suppresses the proliferation of infected cells, thereby promoting the destruction of infected T-cells, as well as inhibiting the establishment of HIV-1 persistent infection (Rogel M E et al., J. Vilol., 69: 882–888, 1995). This activity of Vpr suggests a possibility that Vpr is involved during the transition of the silent period to AIDS.
Vpr has homology in structure as well as in function homologous between simian immunodeficiency virus (SIV) and HIV-1 and HIV-2. Actually, it is shown that in monkeys, Vpr is essential for the onset of AIDS caused by SIV (Hoch J. et al., J. Virol., 69: 4807–4813, 1995).
As described above, Vpr is believed to be involved in the progression of each pathological stage by exerting its various functions at a particular period in the pathological stages from the initial infection to the onset of AIDS. Therefore, compounds that regulate Vpr functions display activities of, for example, 1) inhibiting HIV-1 infection of macrophages and others, which will become reservoirs of HIV in lymphoid tissues such as thymus, and production of mature viruses; 2) inhibiting apoptosis of HIV-infected CD4(+) T-cells, thereby causing the host immune system to induce cytotoxic lymphocytes (CTLs) against infected cells; 3) releasing virus producing cells from their arresting at the G2/M period and reducing the ability of producing viruses; 4) interfering with the destruction of bystander cells (for example, dendritic cells, CD8(+) T-cells, NK, (natural killer) cells) on the basis of cell-membrane impairing activities of Vpr, and as a result, such compounds are likely to prevent the reduction of peripheral un-infected CD4(+) T-cells and to suppress the onset of AIDS and the progression of the AIDS state.
In addition, Vpr arresting of the cell cycle is also reproducible by extracellular adding of recombinant Vpr, and in this case, it is reported that addition of the protein at high concentrations induces cell death (apoptosis) in various cells (Finkel, T. H. et. al., Nature Med., 1: 129–134, 1995). It is said that there is on the order of 2–3 nM Vpr in peripheral blood of actual patients with AIDS, and the effect of this secreted Vpr is likely to cause the destruction of bystander cells as described above in 4). Furthermore, it would be likely that the de-regulation of the cell cycle by Vpr promotes canceration of normal cells, and in combination with reduced immune states, leads to the generation of such AIDS-related tumors as Kaposi's sarcoma.
Recently, attention is being focused on accessory proteins as new targets for creating drugs, with accumulating information on physiological functions of accessory proteins. At present, however, there is no medicament targeting an accessory protein such as Vpr.
With respect to Vpr, the situation has been in confusion: it is already known that when Vpr is overexpressed in various cells, the cell cycle is arrested at the G2/M period, whereas there have been reported contradictory results that when weakly expressed, to the contrary, arresting at the G2/M period is not caused, and also apoptosis does not take place with ease. Recently, however, a cell line that resolves such problems has been established, and screening using these cells has demonstrated that a certain flavonoid (quercetin) inhibits Vpr functions (Shimura M. et. al., Biochem. Biophys. Res. Commun. 261: 308–316, 1999).
Kohyo (National Publication of Translated Version) No. Hei 9-511395 (1997) discloses a biologically active peptide fragment of the Vpr protein of HIV, a pharmaceutical composition comprising such a peptide or its biologically active analogue, an antagonist of the peptide, a pharmaceutical composition comprising the antagonist, and a method for treatment and screening using such a compound and a compositions.
The compounds of the formula (1) according to the present invention are known to have an anti-cancer effect and to be useful as a remedy for malignant tumors (International Publication WO95/27699).